The invention of this application relates to an ink composition for use in a printing apparatus operating on the so-called "ink jet printing" principle. Ink jet printing is a recent development in the art of applying identifying and decorative indicia to a base. In general terms, a fluid ink is forced, under pressure, through a very small orifice in an orifice block which contains a piezoelectric crystal vibrating at high frequency (50-100,000 vibrations per second) causing the ink passing through the orifice to be broken into minute droplets equal in number to the crystal vibrations. The minute droplets are passed through a charging area where individual droplets receive an electrical charge in response to a video signal, the amplitude of the charge being dependent on the amplitude of the video signal. The droplets then pass through an electrical field of fixed intensity, causing a varied deflection of the individual droplets dependent on the intensity of the charge associated therewith, after which the deflected drops are allowed to infringe on the base medium which is to receive the decorative or informative printed indicia. Apparatus suitable for carrying out the ink jet printing process is described in detail in U.S. Pat. Nos. 3,465,350 and 3,465,351, issued Sept. 2, 1969, and it is in connection with an apparatus and process such as are described in the aforementioned patents that the ink of the present invention is designed to function.
Inks suitable for use in the ink jet printing system, or ink jet inks as they will be referred to hereinafter, have been developed which form satisfactory images on paper substrates or other materials of similar surface character. The problems of printing on a metal surface or a metal surface bearing an organic surface coating such as an epoxy type resin, for example, differ substantially from those of printing on paper, and heretofore no ink has been developed which is compatible with both the requirements of the ink jet printing system and the requirements of metal surface printing as well. It is therefore an object of this invention to provide an ink suitable for ink jet printing on metal cans such as those used for the packaging of foods and beverages. It is a particular object of this invention to provide a jet printing ink for printing indicia on aluminum cans which are to be subsequently submitted to pasteurization. It is to be noted that, in discussing the ink jet printing of metal surfaces in this application, it is intended that metal container surfaces bearing a very thin coating of an organic resinous material, such as the coatings commonly applied to exterior surfaces in the manufacture of metal food and beverage containers, be included.
In order to operate satisfactorily in an ink jet printing system, an ink must display a consistent breakup length, drop velocity and drop charge under set machine operating conditions. To achieve these ends, the ink must meet strict requirements with regard to viscosity and resistivity, solubility and compatibility of component, stability and anti-skinning properties and must readily re-dissolve in a suitable solvent for rapid cleanup of the machine components with a minimum of effort.
It has been determined that the workable range of viscosity of an ink which is to be used in a jet printing apparatus in which the nozzle orifice is 0.003 in. in diameter, must be no more than about 5 cps. at 68.degree. F., with about 1.90-2.0 being the most desirable viscosity level for superior performance. The viscosity may be somewhat higher than the above values if the orifice diameter is increased to 0.005 in., for example, but in any case an ink of less than 10 cps. and preferably less than about 5 cps. at 68.degree. F., is highly desirable. Resistivity may range from somewhat less than 100 ohm-cm. to about 1000 ohm-cm., the most desirable value being between about 150 and 300 ohm-cm. Resistivity in excess of about 1000 ohm-cm. creates problems in obtaining the proper charge on the droplets and therefore the deflectability of the droplets in an electric field is erratically impaired.
The orifice through which the ink must pass is normally in the range of 0.002 in. to 0.005 in. in diameter. In order to prevent plugging of this orifice, it is highly desirable that all components of the ink be in solution in the carrier medium rather than in a colloidal or other suspended state. In any case, the complete ink composition must pass at least a 2 micron filter in order to be satisfactory for use. Further, the ink components must not sludge out or otherwise deposit in any of the transporting lines, the supply tank, the orifice or any other portion of the ink supply system, even though the solvent medium of the ink is subject to a certain amount of evaporation in the ink return system and the supply tank. In other words, the solvent medium must have a reserve solubility for the solute components of the ink in order to prevent any undesired precipitation which could clog or plug the minute jet orifice. The ink must also possess anti-skinning properties to prevent skinning over of the orifice or the tank during periods of shut-down. Any skin formed in such circumstances could then break up into small solid particles which could plug the orifice.
In order to facilitate cleanup of the apparatus after use, the ink components should be readily soluble in a common solvent medium. This will prevent any gradual buildup of ink residues in the system which could result in malfunction.
The ink properties set forth above are primarily established by the requirements of the jet printing apparatus. In addition to these requirements, the ink must possess certain other properties which are specifically related to its intended use in the printing of metal cans and, in particular, coated or uncoated aluminum can bodies intended for the packaging of foodstuffs and beverages.
For example, the ink must properly wet the aluminum can surface on which the printed indicia are to appear. If the ink is of such composition that it fails to readily wet the metal surface, the ink will bead up on the surface and fail to adhere properly to it. In extreme cases, the beaded drops will coalesce into larger droplets which run and make the printing completely unintelligible. The problem is often accentuated by oily or greasy residues left on the metal surface from earlier stages of fabrication of the container. On the other hand, if the ink is of such composition as to wet the metal surface too readily, the ink drops will flatten out and spread by "crawling" on the metal surface, diluting the color intensity of the ink and overlapping the image of adjacent dots and spreading out sufficiently to make the printed image fuzzy and the characters unintelligible.
In addition to the requirement of proper wetting of the metal surface to be printed, the droplets of ink must adhere strongly to the surface, after application and drying, so that the printed matter is resistant to both physical rubbing or abrasive action and also is resistant to moisture. The ability of the ink to form and retain a desired image on a metal surface in the presence of moisture and the ability to resist removal by moisture is of great importance in this application because the metal can surfaces are generally damp before, during and after the printing operation. It is particularly difficult to maintain satisfactory adhesion of the ink to metal cans which are subjected to pasteurization, the combination to mositure and high temperature utilized in this process tending to cause the coloring matter to bleed, and to severely reduce the adherance of the ink to the can body so that it is readily removed by subsequent rubbing or abrasion.